A photographic process for forming a direct positive image without necessitating any reversal processing step or negative film is well known.
Known processes for forming a positive image by using a direct positive silver halide photosensitive material except for special ones can be mainly classified into two types from the viewpoint of practical utilization as will be described below.
In one type of the processes, a previously fogged silver halide emulsion is developed to produce a direct positive image by breaking the fog nuclei (latent image) in the exposed region using solarization or according to Harschel effects.
In the other type of the processes, an un-fogged silver halide emulsion of internal latent image-type is imagewisely exposed, and the surface development is conducted after the fogging process or during the fogging process, to obtain a direct positive image.
The term "silver halide photographic emulsion of internal latent iamge-type" indicates a silver halide photographic emulsion having a photosensitive nucleus mainly inside the silver halide grains so that the latent image is formed mainly inside the grains by the exposure.
As compared with the former processes, the latter processes have generally a higher sensitivity and, therefore, they are suitably used when a high sensitivity is required. The process of the present invention belongs to the latter.
Various techniques have been known in this technical field. Typical examples of them are described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577 and British Pat. Nos. 1,151,363, 1,150,553 and 1,011,062.
According to these known processes, photosensitive materials having a relatively high sensitivity as compared with other ones of the direct positive type can be prepared.
The details of the mechanism of the formation of the direct positive image are described in, for example, T. H. James, "The Theory of The Photographic Process", 4th Edition, Chapter 7, pages 182 to 193 and U.S. Pat. No. 3,761,276.
Namely, it is generally considered that fog nuclei are formed selectively on only the surface of the silver halide grains in an unexposed region by the surface desensitization effect due to so-called internal latent image formed inside the silver halide grains by the first imagewise exposure and then an ordinary surface development process is conducted to form the photographic image (direct positive image) in the unexposed region.
As processes for selectively forming the fog nuclei, there have been known a "photo-fogging" process wherein the second exposure is conducted over the whole surface of the photosensitive layer (see, for example, British Pat. No. 1,151,363) and a "chemical fogging process" wherein a nucleating agent is used. The latter process is described in, for example, Research Disclosure, Vol. 151, No. 15162 (published in Nov., 1976), pages 76 to 78.
A direct positive color image is formed by imagewise exposure of a direct positive silver halide photosensitive material followed by (1) a color development conducted in the presence of a nucleating agent and/or fog light and then (2) desilverization process.
It is known that the desilverization can be accelerated by using a bleach-fixing solution containing both ferric complex salt of aminopolycarboxylic acid and thiosulfate as described in German Pat. No. 866,605. However, when the ferric complex salt of aminopolycarboxylic acid having an essentially weak oxidizing power (bleaching power) is combined with the thiosulfate having a reducing power, the bleaching power of the former is seriously deteriorated. Various processes have been proposed for overcoming the defect of the bleach-fixing solution. They include, for example, a process wherein an iodide or bromide is added as described in British Pat. No. 926,569 and Japanese patent publication No. 53-11,854, and a process wherein triethanolamine is used so that the reaction system contains a high concentration of the ferric complex salt of aminopolycarboxylic acid as described in Japanese Patent Public Disclosure No. 48-95,834. However, the effects of these processes are yet insufficient and unpractical.
The bleach-fixing solution has, as well as the defect of the insufficient desilverization power, another defect that a cyan dye formed in the color development is reduced into a leuco dye and the color development is reduced into a leuco dye and the color reproducibility is seriously deteriorated. Although it has been known that this defect can be overcome by elevating the pH of the bleach-fixing solution as described in U.S. Pat. No. 3,773,510, this process is impractical because the bleaching power is weakened as the pH is elevated. In addition, the nucleating agent is unstable under such a high pH condition and it is liable to be oxidized by air. A process wherein the leuco dye is oxidized with a potassium ferricyanide bleaching solution to recover the cyan dye again after the bleach-fixing is described in U.S. Pat. No. 3,189,452. However, potassium ferricyanide is causative of an environmental pollution as described above and even when the bleaching is conducted after the bleach-fixing, the effect of reducing the amount of remaining silver is scarcely exhibited.
On the other hand, as a process for increasing the bleaching power of the ferric complex salt of aminopolycarboxylate when a photosensitive material comprising a negative emulsion is bleached, a process has been proposed wherein a bleaching accelerator selected from various ones is added to a bleaching bath, bleach-fixing bath or preprocessing bath.
The bleaching accelerators include, for example, mercapto compounds described in U.S. Pat. No. 3,893,858, British Pat. No. 138,842 and Japanes Patent Public Disclosure No. 53-141623; compounds having a disulfido bond as described in Japanese Patent Public Disclosure No. 53-95630; thiazolidine derivatives as described in Japanese Patent Publication No. 53-9854; isothiourea derivatives as described in Japanese Patent Public Disclosure No. 53-94927, thiourea deriviatives as described in Japanese Patent Public Disclosure Nos. 45-8506 and 49-26586; thioamido compounds as described in Japanese Patent Public Disclosure No. 49-42349; and dithiocarbamates as described in Japanese Patent Public Disclosure No. 55-26506.
Although some of the bleaching accelerators have the effect of accelerating the bleaching of the color photosensitive materials comrpising a negative emulsion, the effect of them on the color photosensitive materials comprising a positive emulsion is not always satisfactory and they do not satisfy the requirement of reducing the processing time.